Trisomy of 21st Chromosome was first described by

Options

A. Patau
B. Langdon Down
C. Edward
D. Huntington

Answer

**Step-by-Step Solution:**

1. **Identify the Question**: The question asks who first described the trisomy of the 21st chromosome, which is associated with Down syndrome.

2. **Review the Options**: The options provided in the question need to be analyzed to determine which scientist is credited with this discovery.

3. **Recall Historical Context**: John Langdon Down is known for his work on Down syndrome. He described the condition in 1862, although he did not know about chromosomes or the genetic basis of the disorder at that time.

4. **Define Trisomy**: Trisomy refers to the presence of an extra chromosome. In the case of Down syndrome, there are three copies of the 21st chromosome instead of the normal two.

5. **Connect Trisomy to Down Syndrome**: Down syndrome is specifically caused by the trisomy of the 21st chromosome, leading to various physical and developmental characteristics.

6. **Conclude the Answer**: Based on historical records, John Langdon Down is recognized as the first to describe this condition, which is now understood to be caused by the trisomy of the 21st chromosome.

7. **Select the Correct Option**: Therefore, the correct answer is option B, Langdon Down.

**Final Answer**: Langdon Down was the first to describe the trisomy of the 21st chromosome.

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